JOHANNESBURG (Reuters) - South African Olympic and Paralympic track star Oscar Pistorius will face two additional gun-related charges at his trial for the murder of his girlfriend due to start in March next year, prosecutors said on Tuesday.

Pistorius, who is on bail, says he acted in self defence mistaking his model girlfriend Reeva Steenkamp for an intruder when he shot and killed her on Valentine's Day at his upmarket home in February. She was shot in the head, arm and hip.

The additional charges related to incidents prior to the February shooting in which Pistorius was alleged to have fired a gun in a public space. This will form part of the prosecution's overall murder case against Pistorius.

Prosecutors have portrayed Pistorius as a cold-blooded killer and said they were confident that their case, which will have to rely heavily on forensics and witnesses who said they heard shouting before the shots, would stand up to scrutiny.

"The new charges relate to the contravention of the Firearm Control Act, but I cannot go into the details," Nathi Mncube, spokesman for South Africa's National Prosecuting Authority told Reuters. He could not say if the additional charges might delay the existing March 3 start date for the trial.

The 26-year-old double-amputee known as "Blade Runner" for the prostheses he wears in competition appeared in court in August when his trial date was set for next year. A lesser charge of having 38 rounds of unlicensed ammunition at his home was also presented on that occasion.

South African media have reported one previous incident in which Pistorius allegedly fired a gun at a restaurant and another alleged incident in which he fired shots through a car sunroof while in the company of a former girfriend.

Pistorius was born without fibulas and had both his legs amputated below the knees before he turned one year old.

He was one of the most celebrated athletes of the 2012 Olympics and Paralympics in London, progressing to the Olympic 400-metre semi-final and winning Paralympic gold over the same distance.

His arrest and upcoming trial have riveted South Africa and made headlines around the world and, if convicted, he could face life in prison. (Reporting by Peroshni Govender; Editing by Pascal Fletcher)